fix(ux): land quick-win UX audit fixes across settings, exports, toasts, nav

Batch of safe, isolated fixes from the 10-surface UX audit:

- settings: Auto-shield / Save-z-transactions toggles now persist on change
  (they discarded CB()'s return and relied on a Save button that only renders
  on the About tab, so the choice was silently lost on relaunch).
- exports: show the resolved Platform::getConfigDir() instead of a hardcoded
  ~/.config path (wrong on Windows/macOS) in both the all-keys and CSV
  dialogs; all-keys also reveals the file path on a partial export.
- notifications: error/warning toasts last 9s/7s (were 4s/3.5s) so failures
  are readable.
- qr popup / tx-details: copy-to-clipboard now confirms with a toast.
- about: Escape-to-close + readable heading contrast via OnSurfaceMedium().
- sidebar: suppress single-key nav (J/K/arrows/[) while an overlay dialog or
  ImGui popup is open, so it no longer switches the tab behind a modal.
- i18n: strip the stale hardcoded path out of file_save_location in i18n.cpp
  and all eight res/lang/*.json locales (additive, one key each).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/ui/windows/transaction_details_dialog.cpp

@@ -10,6 +10,7 @@
 #include "../theme.h"
 #include "../schema/ui_schema.h"
 #include "../material/draw_helpers.h"
+#include "../notifications.h"
 #include "imgui.h"
 #include <ctime>
 
@@ -126,6 +127,7 @@ void TransactionDetailsDialog::render(App* app)
         ImGui::SameLine();
         if (material::StyledButton((std::string(TR("grpb_copy")) + "##TxID").c_str(), ImVec2(copyBtn.width, 0), S.resolveFont(copyBtn.font))) {
             ImGui::SetClipboardText(tx.txid.c_str());
+            Notifications::instance().info(TR("copied_to_clipboard"));
         }
 
         ImGui::Spacing();
@@ -152,6 +154,7 @@ void TransactionDetailsDialog::render(App* app)
             ImGui::SameLine();
             if (material::StyledButton((std::string(TR("grpb_copy")) + "##Addr").c_str(), ImVec2(copyBtn.width, 0), S.resolveFont(copyBtn.font))) {
                 ImGui::SetClipboardText(tx.address.c_str());
+                Notifications::instance().info(TR("copied_to_clipboard"));
             }
         }